The particles from filamentous bacteriophages are generally considered to be flexible. This assumption originally arose from the observation of electron micrographs where the virions appeared as long and randomly bent filaments. How accurately does this aspect of filamentous viruses represent the behaviour in solution? While the conditions of microscopic experimentation are very different from those of solution or the natural environment, other observations may correspond to more rigid particles. Hydrodynamical measurements are a help to ascertain the actual degree of flexibility of these particles in the more real three dimensional solution conditions. Employing sedimentation equilibrium and sedimentation velocity data for bacteriophage fd, and mathematical modelling, we explore the behaviour of these particles here. The relative insensitivity of the data pair (sedimentation coefficient–translational diffusion coefficient) so obtained is confirmed and the superiority of the conformational information contained in other parameters, such as the rotational diffusion coefficient and the intrinsic viscosity. Despite this lack of sensitivity, the straighter model conformations satisfy better the experimental data, which can be an indication of a relatively rigid viral particle.
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